The present invention generally relates to mode signalling in the field of communication systems and, more particularly, to mode signalling involving multiple modulation and coding schemes, link adaptation and incremental redundancy in digital communication systems.
The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems, have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. One technique to achieve these objectives involved changing from systems wherein analog modulation was used to impress data onto a carrier wave, to systems wherein digital modulation was used to impress the data on carrier waves.
In order to provide various communication services, a corresponding minimum user bit rate is required. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques for speech coding, channel coding, modulation and resource allocation (e.g., for a TDMA system, the number of assignable time slots per call and for a CDMA system, the number of codes assigned to a call).
Considering first the impact of modulation, different digital communication systems have conventionally used a variety of linear and non-linear modulation schemes to communicate voice or data information. These modulation schemes include, for example, Gaussian Minimum Shift Keying (GMSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. Typically, each communication system operates using a single modulation scheme for transmission of information under all conditions. For example, ETSI originally specified the GSM standard to communicate control, voice and data information over links using a GMSK modulation scheme to provide transmission and retransmission of information.
Depending on the modulation scheme used by a particular system, the throughput of a packet transmission scheme deteriorates differently as C/I levels decrease. For example, modulation schemes may use a different number of values or levels to represent information symbols. The signal set, i.e., amplitude coefficients, associated with QPSK, an exemplary lower level modulation (LLM) scheme, are illustrated in FIG. 1(a). By way of comparison, 16QAM is a higher level modulation (HLM) scheme having the signal set depicted in FIG. 1(b).
As can be seen in FIGS. 1(a) and 1(b), the minimum Euclidean distance between the coefficients in the LLM scheme is greater than the minimum Euclidean distance between coefficients in the HLM scheme for the same average signal power, which makes it easier for receive signal processing to distinguish between modulation changes in the LLM scheme. Thus, LLM schemes are more robust with respect to noise and interference, i.e., require a lower carrier-to-interference (C/I) level to achieve acceptable received signal quality. HLM schemes, on the other hand, provide greater user bit rates, e.g., 16QAM provides twice the user bit rate of QPSK, but require higher C/I levels.
More recently, however, dynamic adaptation of the modulation used for transmission in radiocommunication systems types has been considered as an alternative that takes advantage of the strengths of individual modulation schemes to provide greater user bit rates and/or increased resistance to noise and interference. An example of a communication system employing multiple modulation schemes is found in U.S. Pat. No. 5,577,087. Therein, a technique for switching between 16QAM and QPSK is described. The decision to switch between modulation types is made based on quality measurements, however this system employs a constant user bit rate which means that a change in modulation scheme also requires a change in channel bit rate, e.g., the number of timeslots used to support a transmission channel.
In addition to modulation schemes, digital communication systems also employ various techniques to handle erroneously received information, which techniques also affect the bit rate experienced by the user. Generally speaking, these techniques include those which aid a receiver to correct the erroneously received information, e.g., forward error correction (FEC) techniques, and those which enable the erroneously received information to be retransmitted to the receiver, e.g., automatic retransmission request (ARQ) techniques. FEC techniques include, for example, convolutional or block coding of the data prior to modulation. FEC coding involves representing a certain number of data bits using a certain number of code bits. Thus, it is common to refer to convolutional codes by their code rates, e.g., 1/2 and 1/3, wherein the lower code rates provide greater error protection but lower user bit rates for a given channel bit rate.
ARQ techniques involve analyzing received blocks of data for errors and requesting retransmission of blocks which contain errors. Consider, for example, the block mapping example illustrated in FIG. 2 for a radiocommunication system operating in accordance with the Generalized Packet Radio Service (GPRS) optimization which has been proposed as a packet data service for GSM. Therein, a logical link control (LLC) frame containing a frame header (FH), a payload of information and a frame check sequence (FCS) is mapped into a plurality of radio link control (RLC) blocks, each of which include a block header (BH), information field, and block check sequence (BCS), which can be used by a receiver to check for errors in the information field. The RLC blocks are further mapped into physical layer bursts, i.e., the radio signals which have been GMSK modulated onto the carrier wave for transmission. In this example, the information contained in each RLC block can be interleaved over four bursts (timeslots) for transmission.
When processed by a receiver, e.g., a receiver in a mobile radio telephone, each RLC block can, after demodulation, be evaluated for errors using the block check sequence and well known cyclic redundancy check techniques. If there are errors, then a request is sent back to the transmitting entity, e.g., a base station in a radiocommunication system, denoting the block to be resent using predefined ARQ protocols. The variation of both modulation and FEC schemes (referred to herein jointly as “modulation/coding schemes” or “MCS”) to provide link adaptation in conjunction with ARQ is described, for example, in U.S. patent application Ser. No. 08/921,318, entitled “A Method for Block ARQ with Reselection of FEC Coding and Modulation”, filed on Aug. 29, 1997, the disclosure of which is incorporated here by reference.
Strengths and weaknesses of these two error control schemes can be balanced by combining FEC and ARQ techniques. Such combined techniques, commonly referred to as hybrid ARQ techniques, permits correction of some received errors using the FEC coding at the receiver, with other errors requiring retransmission. Proper selection of FEC coding schemes with ARQ protocols thus results in a hybrid ARQ technique having greater reliability than a system employing a purely FEC coding scheme with greater throughput than a system employing a purely ARQ-type error handling mechanism.
An example of a hybrid ARQ scheme can be found in GPRS. The GPRS optimization provides four FEC coding schemes (three convolutional codes of different rate and one uncoded mode). After one of the four coding schemes is selected for a current LLC frame, segmentation of this frame to RLC blocks is performed. If an RLC block is found to be erroneous at the receiver (i.e., it has errors which cannot be corrected) and needs to be retransmitted, the originally selected FEC coding scheme is used for retransmission, i.e., this system employs fixed redundancy for retransmission purposes. The retransmitted block may be combined with the earlier transmitted version in a process commonly referred to as soft combining in an attempt to successfully decode the transmitted data.
Another proposed hybrid ARQ scheme, sometimes referred to as incremental redundancy or type-II hybrid ARQ, provides for additional redundant bits to be transmitted if the originally transmitted block cannot be decoded. This scheme is conceptually illustrated in FIG. 3. Therein, three decoding attempts are made by the receiver. First, the receiver attempts to decode the originally received data block (with or without redundancy). Upon failure, the receiver then receives additional redundant bits R1, which it uses in conjunction with the originally transmitted data block to attempt decoding. As a third step, the receiver obtains another block of redundant information R2, which it uses in conjunction with the originally received data block and the block of redundant bits R1 to attempt decoding for a third time. This process can be repeated until successful decoding is achieved.
As compared with link adaptation, incremental redundancy does not require that link quality estimates be transmitted or used. However, one problem with this technique is the large memory requirement associated with storing the data block (and possibly additional blocks of redundant bits) until a successful decode occurs, which storage is needed since the subsequently transmitted redundancy blocks (e.g., R1 and R2) cannot be independently decoded to give the same performance as if combined decoding was used. The storage requirements are further increased if the receiver stores a multi-bit soft value associated with each received bit, the soft values indicating a confidence level associated with the decoding of the received bit.
Many variations and combinations of these techniques are possible. For example, it is possible to combine link adaptation with incremental redundancy. This results in an incremental redundancy scheme wherein the MCS of the first transmission can be varied, e.g., such that the first transmission is made using some channel coding or not the least robust modulation. In such a combination, the MCS can be changed for many reasons, e.g., to reduce the number of retransmissions or delay or to dynamically adapt to changes in memory requirements.
MCS changes may or may not be based solely on reported link quality estimates. For example, when incremental redundancy is used and the receiver has limited memory it may be beneficial to increase the robustness of the MCS even though (in a system with unlimited memory) it would decrease throughput. Consider the following scenario. Using a less robust MCS, the number of required retransmissions for successful incremental redundancy combination will be higher. This, in turn, requires a lot of memory. If the receiver runs out of memory, it will begin to discard received blocks that have previously been stored for later incremental redundancy combination. Since the information transmitted using the relatively unrobust MCS probably relies in part upon incremental redundancy combining to achieve acceptable decoding performance, the result may be significantly degradation in received signal quality. Hence it may be better under such circumstances to dynamically increase the robustness of the transmission's MCS, e.g., when the receiver starts to run out of memory.
Yet another factor which further complicates this process is the possibility of changing the MCS associated with blocks that are being retransmitted. If link adaptation is used without incremental redundancy, then changing the MCS for retransmissions may be very desirable based upon the measured link quality. On the other hand, if incremental redundancy is employed, using a different MCS may make it impossible to combine the retransmitted block with the originally transmitted block. However, if the link quality changes significantly it still may be desirable to change the MCS even if some of the earlier transmitted data blocks cannot be used in the redundancy combining process at the receiver.
Thus it can be seen that there are many challenges associated with optimizing the manner in which these various techniques are employed. To enable dynamic changing of the MCS during a connection, some form of overhead signalling is necessary between the transmitter and receiver. Conventionally, overhead signalling associated with MCS changes has been performed as illustrated in FIGS. 4(a) and 4(b). In FIG. 4(a), control of MCS changes resides with the transmitting entity 40. Then, the receiving entity 42 makes quality measurements on signals transmitted on the forward link 44. The receiving entity 42 transmits the quality measurements on the reverse link 46 back to the transmitting entity 40, which then determines an appropriate MCS for subsequent block transmissions. This information is then forwarded to the receiving entity 42 so that it is prepared for any changes in the MCS.
Alternatively, control of MCS changes may reside with the receiving entity 42 as shown in FIG. 4(b). Then, the receiving entity 42 makes quality measurements on the forward link as in FIG. 4(a). However, instead of transmitting the quality measurements to the transmitting entity 40, the receiving entity determines if any MCS changes are desirable and forwards such information to the transmitting entity on the reverse link 46.
Both of these conventional signalling techniques have certain drawbacks in the context of systems which can employ both link adaptation and incremental redundancy. Specifically, the signalling technique of FIG. 4(a) suffers from the drawback that the transmitter 40, which is controlling the MCS changes, has no knowledge of the receiver 42's memory status. As mentioned above, without this knowledge, the transmitter cannot properly select an MCS which is appropriate based on both the link quality and the limited memory available to support incremental redundancy combining at the receiver.
Similarly, the conventional technique of FIG. 4(b) also suffers from drawbacks. For example, the conventional MCS choice information transmitted on the reverse link 46 applied only to originally transmitted blocks. As described above, however, it may be desirable for the MCS for originally transmitted blocks and retransmitted blocks to be different.
Accordingly, it would be desirable to provide enhanced techniques for controlling the operation of a radiocommunication system involving link adaptation and incremental redundancy.